Applicants claim priority of Japanese patent applications, Ser. No. 2001-374,117, filed Dec. 7, 2001, Ser. No. 2001-374,118, filed Dec. 7, 2001 and Ser. No. 2001-374,119 filed Dec. 7, 2001.
The present invention relates to a rotary throttle valve carburetor for an internal combustion engine, and more particularly to such a carburetor having a starting device.
The conventional rotary throttle valve-type carburetor is designed so that turning of the throttle valve causes a needle to be moved up and down to adjust the extent to which a fuel nozzle is open. In low temperatures when the engine is cold, frictional resistance in the engine is high. Therefore, the engine is hard to start, and even if the engine is started its idle operation is unstable.
As shown in FIG. 51, a conventional rotary throttle valve-type carburetor has a carburetor body 5 provided with a cylindrical valve chamber 6 perpendicular to an air intake passage (extending vertically relative to the paper surface) a throttle valve 1 having a throttle hole 2 is rotatably and vertically moveably fitted in the valve chamber 6. A valve shaft 1a of the throttle valve 1 extends through a lid plate 21 for closing the valve chamber 6, and a throttle valve lever 22 is connected to the upper end of the valve shaft 1a. A swivel 23 for connecting a remote control cable is rotatably supported on one end of the throttle valve lever 22, whereas a cam portion 22a is provided on the other end of the throttle valve lever 22. A peripheral cam groove with a depth that becomes gradually shallower in a direction of rotation corresponding to an increased throttle valve opening is provided in the lower surface of the cam portion 22a and a follower supported on the lid plate 21 is engaged with the cam groove to thereby constitute a cam mechanism.
Fuel is taken into a fuel nozzle of a fuel supply pipe 4 projecting toward the throttle hole 2 via a check valve and a fuel jet. In FIG. 51, the throttle valve 1 is in a fully open position, and the throttle hole 2 and the air intake passage are substantially registered or coincident in an axial direction. A needle 3 projecting downward from the throttle valve 1 is inserted into the fuel supply pipe 4.
In operation, to increase the speed and/or power of the engine, the throttle valve lever 22 is turned or rotated in an accelerating direction against the force of a spring to increase the extent to which the throttle hole 2 is open relative to the air intake passage. At the same time, the needle 3 is moved up by the aforementioned cam mechanism to increase the extent to which the fuel nozzle is open.
A start shaft 32 is fitted into a guide tube 53 formed integral with the lid plate 21, and when the start shaft 32 is turned by means of a start lever 31, a cam surface 52 formed on the end portion of the start shaft 32 lifts up the throttle lever 22 so as to increase the quantity of fuel. A pin 51 on the guide tube 53 is engaged with an annular groove of the start shaft 32 to retain the start shaft 32 in the guide tube 53.
In a small engine for a work tool provided with a centrifugal clutch and the aforementioned rotary throttle valve-type carburetor, when the airflow through the carburetor is increased sufficiently over the calibrated air flow for idle engine operation (thereby increasing the engine rpm at idle), the centrifugal clutch can become connected so that a tool driven by the engine is actuated, which may be undesirable. Accordingly, the airflow when the engine is started has to be set so that the speed (rpm) of the engine is slightly faster than the calibrated idle setting, but not so high as to engage the clutch.
However, after the break-in period of the engine, the set idle speed becomes faster than the value set after assembly at the factory. At this time, when the idling speed is adjusted to a proper value the increase in airflow at the start of the engine as adjusted by the start fuel increasing mechanism, can place the speed of the engine out of its desired range.
A starting device for a rotary throttle valve-type carburetor enables adjustment of the quantity of air and fuel delivered to an engine to facilitate the cold start of the engine. In one embodiment, the starting device has an axially slidable sleeve fitted into a guide tube supported on a lid plate for closing a valve chamber of a carburetor body. A pin extending through the sleeve is engaged with an axial slit of the guide tube. A first projecting part extends outwardly from the guide tube and a second projecting part extends outwardly from the sleeve, and an idling adjusting bolt extends through the second projecting part and is threadedly fitted in the first projecting part. A start shaft having a helical groove in engagement with the pin is fitted into the sleeve, and has an actuator comprising at least in part a flat cam surface for engagement with a cam plate provided on a valve shaft of a throttle valve. A push rod for engagement with a side wall surface provided on the valve shaft is formed on the end of the start shaft. When the start shaft is rotated, the cam surface engages and lifts the throttle valve to increase fuel flow, and the push rod rotates the throttle valve to further open it and increase the air flow. By adjusting the position of the cam surface and the push rod relative to the throttle valve, the extent of the increase in fuel flow and air flow can be adjusted to provide a desired fuel and air mixture to facilitate starting the engine.
In another embodiment, a start shaft is threaded in a boss portion formed on the lid plate. A cam surface is formed on the end portion of the start shaft, a push rod is threaded in the start shaft, and a protrusion is formed on the lower surface of a throttle valve lever connected to a valve shaft of the throttle valve. When the start shaft is rotated, a throttle valve lever is lifted up by the cam surface, and the protrusion on the throttle valve lever is pushed by the axial movement of said start shaft and push rod to turn or rotate the throttle valve lever.
In another embodiment, the actuator comprises an eccentric push rod with a cam surface to both lift and rotate the throttle valve lever. Several other embodiments of carburetors with starting assemblies are disclosed herein.